Using a volumetric spot light, it is possible to hide totally the elements that the light doesn't reach?
You can change your spot with a 0 bounce (direct light only). The light won't bounce outside you volume anymore.



This will affect the lighting of your main object (no indirect light), so you can move the 2 others to another layer and add a "no bounce" Spotlight (duplicated or parented if animation is needed) and composite render layers. Your main object (with a spot with more than 0 max bounce) will have a nice direct and indirect lighting and the others won't be affected by bounces.

This setting was added in blender 2.73 official release (release notes)